Somatic cell genetic analysis has permitted the detection, characterization and genetic localization of a number of mammalian cellular genes which participate in endogenous type C virus expression. Six classes of genes are under study: (1) Endogenous cellular DNA sequences homologous to type C viral RNA; (2) integration sites of retroviruses in mammalian chromosomes; (3) viral cell surface receptors; (4) restriction genes which delimit viral replication; (5) enzyme structural genes; (6) cell surface antigens including MHC. Examples of each of these gene classes have been detected and mapped in human and feline model systems. The allozyme genetic signature as a definitive characteristic of cultured cells has been applied to monitors of cell contamination, quality control of in vitro transformation and identification of tissue origin in human embryonic development and following amnioscentesis. The population genetic structure of the outbred Swiss and the feral Lake Casitas mouse populations has been described.